1. Field of the Invention
The invention relates to an F-theta objective as can be used in a scan apparatus for laser material processing.
2. Description of the Background Art
An F-theta objective focuses a laser beam, which is incident in a manner such that it scans over a scan-angle region +/−θ with respect to the optical axis of the F-theta objective, into a flat image field, wherein, within this scan-angle region, the ratio of scan angle and distance of the point of incidence of the laser beam from the optical axis in the image field follows a linear function. That is to say that a laser beam scanning at a constant angular velocity generates a focal point in the image field, which focal point moves at a constant speed. Here, the size of the focal point should be constant at each location in the image field.
The size of the focal point is determined in dependence on the purpose of the laser material processing, e.g. writing, coating removal or cutting.
Owing to the wavelength-dependent refraction of the laser beam as it passes through the F-theta objective, F-theta objectives are corrected, in order to achieve a high focal point quality, to the wavelength of the processing laser beam used, that is to say the objective is calculated such that it has, for an image field of prespecified size within a permissible temperature tolerance for a prespecified wavelength and a prespecified laser beam diameter, no or only very slight optical aberrations which result in a distinct change in the size of the focal point. In particular for use in laser material processing, F-theta objectives have a large image field and a large total focal length.